The present invention relates to a pneumatic tire improved in run-flat performance.
Recently, there have been marketed run-flat tires whose sidewall is reinforced by a hard rubber layer having a specific cross sectional shape so as to be strong enough to temporarily support the car without the help of air pressure.
In such run-flat tires, the ground pressure greatly increases in the tread shoulder region, and in contrast therewith, the ground pressure in the tread crown greatly decreases. As a result, heat generation and tread rubber wear is accelerated in the tread shoulder region and finally the tire is broken. Further, because of the irregular ground pressure distribution, it is difficult for the flat tire to produce a sufficient cornering power. Therefore, stability during cornering is not good.
It is therefore, an object of the present invention to provide a pneumatic tire, in which run-flat performance such as runable distance is further improved and at the same time cornering stability is also improved.
According to the present invention, a pneumatic tire comprises
a tread portion,
a pair of sidewall portions,
a pair of bead portions,
a carcass extending between the bead portions, and
a tread reinforcement disposed radially outside the carcass in the tread portion and comprising a reinforcing rubber layer and a radially outer reinforcing cord layer,
the radially outer reinforcing cord layer made of cords disposed radially outside the reinforcing rubber layer,
the reinforcing rubber layer extending across the substantially overall tread width and made of a rubber material having a hardness of not less than 70 degrees and a loss tangent of not more than 0.15,
in a tire meridian section, the thickness of the reinforcing rubber layer being in a range of not less than 2 mm at the tire equator and gradually decreasing towards the axial edges of the reinforcing rubber layer.
The tread reinforcement may be formed to include a radially inner reinforcing cord layer between the reinforcing rubber layer and the carcass. In this case, it is preferable that the inner reinforcing cord layer includes a ply of cords laid at an angle of substantially 90 degrees with respect to the tire equator.
The radially outer reinforcing cord layer and/or the radially inner reinforcing cord layer may include a belt ply made of cords laid at an angle of from 10 to 30 degrees with respect to the tire equator.
The outer reinforcing cord layer may include a band ply made of organic fiber cords, whose cord angles are not more than 5 degrees with respect to the tire equator, alone or in combination with the belt ply.
In this invention, the xe2x80x9chardnessxe2x80x9d of rubber means a hardness measured with a type-A durometer according to Japanese Industrial Standard K6253.
The loss tangent is measured under a temperature of 70 degrees C., a frequency of 10 Hz, and a dynamic distortion of plus/minus 2%.
The tread width is defined as the maximum axial width of the ground contacting area under a standard loaded condition in which the tire is mounted on a standard rim and inflated to a standard load and then loaded with a standard load. The standard rim is the xe2x80x9cstandard rimxe2x80x9d specified in JATMA, the xe2x80x9cMeasuring Rimxe2x80x9d in ETRTO, the xe2x80x9cDesign Rimxe2x80x9d in TandRA or the like. The standard pressure is the xe2x80x9cmaximum air pressurexe2x80x9d in JATMA, the xe2x80x9cInflation Pressurexe2x80x9d in ETRTO, the maximum pressure given in the xe2x80x9cTire Load Limits at Various Cold Inflation Pressuresxe2x80x9d table in TandRA or the like. In case of passenger car tires, however, 180 kPa is used as the standard pressure.
The standard load is defined as the xe2x80x9cmaximum load capacityxe2x80x9d in JATMA, 70% of the xe2x80x9cLoad Capacityxe2x80x9d in ETRTO, the maximum value given in the above-mentioned table in TandRA or the like.
Further, a standard unloaded condition is defined such that the tire is mounted on the standard rim and inflated to the standard pressure, but loaded with no tire load.